Acousto-optic variable attenuator with active cancellation of back reflections

ABSTRACT

An acousto-optic filter includes an optical fiber with a first region and a second region. A first acoustic wave generator is coupled to optical fiber. The first acoustic wave generator produces a first acoustic wave that travels in a first direction in the first region. A backward-propagating wave is generated in response to propagation of the first acoustic wave along the optical fiber. The backward-propagating wave travels in an opposite direction relative to the first acoustic wave. A first acoustic wave propagation member is coupled to the optical fiber. A second acoustic wave generator is coupled to the optical fiber at the second region. The second acoustic wave generator produces a second acoustic wave that combines with the backward propagating acoustic wave.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of Ser. No. 09/666,763,pending filed Sep. 21, 2000, which is a continuation-in-part of andclaims the benefit of Ser. No. 60/206,767 filed May 23, 2000, Ser. No.09/666,763 also being a continuation-in-part of Ser. No. 09/571,092filed May 15, 2000, now U.S. Pat. No. 6,253,002 which is a continuationof Ser. No. 09/425,099, now U.S. Pat. No. 6,233,379, filed Oct. 22,1999, which is a continuation-in-part of Ser. No. 09/022,413 filed Feb.12, 1998, now U.S. Pat. No. 6,021,237, which claims priority to KR97-24796 filed Jun. 6, 1997; all of which applications are fullyincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a variable optical attenuator (VOA),and more particularly to an all-fiber acousto-optic tunable intensityattenuator that includes an acoustic wave device that reducesbackward-propagating acoustic waves.

2. Description of Related Art

In modern telecommunication systems, many operations with digitalsignals are performed on an optical layer. For example, digital signalsare optically amplified, multiplexed and demultiplexed. In long fibertransmission lines, the amplification function is performed by ErbiumDoped Fiber Amplifiers (EDFA's). The amplifier is able to compensate forpower loss related to signal absorption, but it is unable to correct thesignal distortion caused by linear dispersion, 4-wave mixing,polarization distortion and other propagation effects, and to get rid ofnoise accumulation along the transmission line. For these reasons, afterthe cascade of multiple amplifiers the optical signal has to beregenerated every few hundred kilometers. In practice, the regenerationis performed with electronic repeaters using optical-to-electronicconversion. However to decrease system cost and improve its reliabilityit is desirable to develop a system and a method of regeneration, orsignal refreshing, without optical to electronic conversion. An opticalrepeater that amplifies and reshapes an input pulse without convertingthe pulse into the electrical domain is disclosed, for example, in theU.S. Pat. No. 4,971,417, Radiation-Hardened Optical Repeater”. Therepeater comprises an optical gain device and an optical thresholdingmaterial producing the output signal when the intensity of the signalexceeds a threshold. The optical thresholding material such aspolydiacetylene thereby performs a pulse shaping function. The nonlinearparameters of polydiacetylene are still under investigation, and itsability to function in an optically thresholding device has to beconfirmed.

Another function vital to the telecommunication systems currentlyperformed electronically is signal switching. The switching function isnext to be performed on the optical level, especially in the WavelengthDivision Multiplexing (WDM) systems. There are two types of opticalswitches currently under consideration. First, there are wavelengthinsensitive fiber-to-fiber switches. These switches (mechanical, thermoand electro-optical etc.) are dedicated to redirect the traffic from oneoptical fiber to another, and will be primarily used for networkrestoration and reconfiguration. For these purposes, the switching timeof about 1 msec (typical for most of these switches) is adequate;however the existing switches do not satisfy the requirements for lowcost, reliability and low insertion loss. Second, there are wavelengthsensitive switches for WDM systems. In dense WDM systems having a smallchannel separation, the optical switching is seen as a wavelengthsensitive procedure. A small fraction of the traffic carried by specificwavelength should be dropped and added at the intermediate communicationnode, with the rest of the traffic redirected to different fiberswithout optical to electronic conversion. This functionality promisessignificant cost saving in the future networks. Existing wavelengthsensitive optical switches are usually bulky, power-consuming andintroduce significant loss related to fiber-to-chip mode conversion.Mechanical switches interrupt the traffic stream during the switchingtime. Acousto-optic tunable filters, made in bulk optic or integratedoptic forms, (AOTFs) where the WDM channels are split off by coherentinteraction of the acoustic and optical fields though fast, less thanabout 1 microsecond, are polarization and temperature dependent.Furthermore, the best AOTF consumes several watts of RF power, hasspectral resolution about 3 nm between the adjacent channels (which isnot adequate for current WDM requirements), and introduces over 5 dBloss because of fiber-to-chip mode conversions.

Another wavelength-sensitive optical switch may be implemented with atunable Fabry Perot filter (TFPF). When the filter is aligned to aspecific wavelength, it is transparent to the incoming optical power.Though the filter mirrors are almost 100% reflective no power isreflected back from the filter. With the wavelength changed or thefilter detuned (for example, by tilting the back mirror), the filterbecomes almost totally reflective. With the optical circulator in frontof the filter, the reflected power may be redirected from the incidentport. The most advanced TFPF with mirrors built into the fiber and PZTalignment actuators have only 0.8 dB loss. The disadvantage of thesefilters is a need for active feedback and a reference element forfrequency stability.

A VOA is an opto-mechanical device capable of producing a desiredreduction in the strength of a signal transmitted through an opticalfiber. Ideally, the VOA should produce a continuously variable signalattenuation while introducing a normal or suitable insertion loss andexhibiting a desired optical return loss. If the VOA causes excessivereflectance back toward the transmitter, its purpose will be defeated.

Due to the limitations of related art, there is a need for an AOTF withreduced backward-propagating waves that are created in response topropagation of an acoustic wave along an optical fiber of the AOTF.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a VOA withreduced backward-propagating acoustic waves.

These and other objects of the present invention are achieved in anacousto-optic filter that includes an optical fiber with a first regionand a second region. A first acoustic wave generator is coupled tooptical fiber. The first acoustic wave generator produces a firstacoustic wave that travels in a first direction in the first region. Abackward-propagating wave is generated in response to propagation of thefirst acoustic wave along the optical fiber. The backward-propagatingwave travels in an opposite direction relative to the first acousticwave. A first acoustic wave propagation member is coupled to the opticalfiber. A second acoustic wave generator is coupled to the optical fiberat the second region. The second acoustic wave generator produces asecond acoustic wave that reduces a magnitude of the backwardpropagating acoustic wave.

In another embodiment of the present invention, an acousto-optic filterincludes an optical fiber with a first region and a second region and afirst acoustic wave generator coupled to the optical fiber. The firstacoustic wave generator produces a first acoustic wave that travels in afirst direction in the first action. In response to propagation of thefirst acoustic wave along the optical fiber a backward-propagating waveis generated. The backward-propagating wave travels in an oppositedirection relative to the first acoustic wave. An acoustic damper iscoupled to the optical fiber at the non-interaction region. The acousticdamper includes a proximal end with a taper configuration that reduces apower of the backward-propagating wave in a range of 20 to 30 dB orless.

In another embodiment of the present invention, an acousto-optic filteris provided that includes an optical fiber with a first region and asecond region. A first acoustic wave generator is coupled to the opticalfiber. The first acoustic wave generator produces a first acoustic wavethat travels in a first direction in the first action. Abackward-propagating wave is created in response to propagation of thefirst acoustic wave along the optical fiber. The backward-propagatingwave travels in an opposite direction relative to the first acousticwave. A first acoustic wave propagation member is coupled to the opticalfiber. An acoustic damper coupled to the non-interaction region of theoptical fiber. A second acoustic wave generator is coupled to theacoustic damper. The second acoustic wave generator produces a secondacoustic wave that reduces a magnitude of the backward propagatingacoustic wave.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1(a) is a schematic diagram of one embodiment of an AOTF of thepresent invention.

FIG. 1(b) is a cross-sectional view of the optical fiber of the FIG. 1AOTF.

FIG. 2 is a cross-sectional view of one embodiment of an acoustic wavepropagation member that can be used with the AOTF of FIG. 1.

FIG. 3(a) is a cross-sectional view illustrating one embodiment of aninterface created between an optical fiber and a channel formed in anacoustic wave propagation member of the FIG. 1 AOTF.

FIG. 3(b) is a cross-sectional view illustrating an embodiment of aninterface between an optical fiber and a channel formed in an acousticwave propagation member of the FIG. 1 AOTF where a bonding material isused.

FIG. 4 is a schematic diagram of one embodiment of an AOTF of thepresent invention with an acoustic damper.

FIG. 5 is a cross-sectional view of one embodiment of an index profileof an optical fiber, useful with the AOTF of FIG. 1, that has a doublingcladding.

FIG. 6 is a cross-sectional view of an optical fiber with sections thathave different diameters.

FIG. 7 is a cross-sectional view of an optical fiber with a taperedsection.

FIG. 8 is a perspective view of one embodiment of an AOTF of the presentinvention that includes a heatsink and two mounts.

FIG. 9 is a perspective view of one embodiment of an AOTF of the presentinvention with a filter housing.

FIG. 10 is a block diagram of an optical communication system with oneor more AOTF's of the present invention.

FIG. 11 is a schematic view showing the structure of an acousto-optictunable filter according to one embodiment of the present invention.

FIG. 12 is a graph showing the coupling and transmittance of the filterof FIG. 1.

FIG. 13 is a graph showing the transmittance of the filter of FIG. 11.

FIG. 14 is a graph showing the center wavelength of filter of FIG. 1 asa function of the frequency applied to the acoustic wave generator.

FIGS. 15(a)-(d) are graphs illustrating the transmissions of the filterof FIG. 11 when multiple frequencies are applied to the acoustic wavegenerator.

FIGS. 16(a)-(b) are graphs showing the transmittance characteristics ofthe filter of FIG. 11 when varying an electric signal with a threefrequency component applied to the filter.

FIGS. 17(a)-(d) are graphs for comparing the mode convertingcharacteristic of the filter according to an embodiment of the presentinvention with that of a conventional wavelength filter.

FIGS. 18(a) illustrate one embodiment of a transmission spectrum of theFIG. 11 filter.

FIG. 18(b) illustrates the measured and the calculated centerwavelengths of the notches as a function of acoustic frequency of anembodiment of the FIG. 11 filter.

FIG. 19 illustrates two examples of configurable spectral profiles withspectral tilt from the FIG. 11 filter.

FIG. 20(a) is a filter assembly that includes two filters of FIG. 11that are in series.

FIG. 20(b) is a schematic diagram of a dual-stage EDFA with a filter ofFIG. 20(a).

FIG. 21(a) is a graph of gain profiles of an EDFA with the filter ofFIG. 20(a).

FIG. 21(b) is a graph illustrating filter profiles that produced theflat gain profiles shown in FIG. 21(a).

FIG. 21(c) is a graph illustrating filter profiles of the FIG. 20(a)filter assembly.

FIGS. 22(a) and 22(b) are graphs illustrating the polarizationdependence of one embodiment of the filter of the present invention.

FIG. 23 illustrates one embodiment of the present invention from FIG. 4that has a reduction with a lower polarization dependent loss.

FIGS. 24(a) and 24(b) are graphs illustrating the polarization dependentloss profile of one embodiment of the invention, from FIG. 4, when thefilter is operated to produce 10-dB attenuation at 1550 nm.

FIG. 25 illustrates an embodiment of the invention with two of thefilters of FIG. 1.

FIG. 26 is a graph illustrating the effects of a backward acousticreflection at the damper of one embodiment of the present invention fromFIG. 4.

FIG. 27(a) is a graph illustrating, in one embodiment of FIG. 4, themodulation depth at 10-dB attenuation level at both first- andsecond-harmonics of the acoustic frequency.

FIG. 27(b) is a graph illustrating the modulation depth of first- andsecond-harmonics components from FIG. 27(a).

FIG. 28 is a schematic diagram of one embodiment of a VOA assembly ofthe present invention with a feedback loop.

FIG. 29 is a schematic diagram of another embodiment of a VOA of thepresent invention with a demultiplexer and a multiplexer.

FIG. 30 is a schematic diagram of an embodiment of a channel equalizerof the present invention using a single router.

FIG. 31 is a schematic diagram of another embodiment of a channelequalizer of the present invention using multiple routers.

FIG. 32 is a schematic diagram of an embodiment of the present inventionwith a second acoustic wave generator.

FIG. 33 is a schematic diagram of an embodiment of the present inventionwith a feedback loop coupled to the second acoustic generator of theFIG. 32 device.

FIG. 34 is a schematic diagram of an embodiment of the present inventionthat includes an acoustic damper with a tapered proximal end.

FIG. 35 is a schematic diagram of an embodiment of the present inventionwith an acoustic damper and a second acoustic wave generator.

DETAILED DESCRIPTION

FIG. 1 illustrates one embodiment of an AOTF (hereafter filter 10) ofthe present invention. An optical fiber 12 has a longitudinal axis, acore 14 and a cladding 16 in a surrounding relationship to core 14.Optical fiber 12 can be a birefringent or non-birefringent single modeoptical fiber and a multi-mode fiber. Optical fiber 12 can have,multiple modes traveling within the fiber such as core to core, core tocladding and polarization to polarization, multiple cladding modes and asingle core mode guided along core 14, support core to cladding modesand multiple cladding modes. Optical fiber 12 can provide multiple mode,including but not limited to fundamental and cladding mode, propagationalong a selected length of optical fiber 12. Alternatively, opticalfiber 12 is a birefringent single mode fiber that does not have multiplecladding modes and a single core mode. In one embodiment, optical fiber12 is tensioned. Sufficient tensioning can be applied in order to reducelosses in a flexure wave propagated in optical fiber 12.

The core of optical fiber 12 is substantially circular-symmetric. Thecircular symmetry ensures that the refractive index of the core mode isessentially insensitive to the state of optical polarization. Incontrast, in hi-birefringent single mode fibers the effective refractiveindex of the core mode is substantially different between two principalpolarization states. The effective refractive index difference betweenpolarization modes in high birefringence single mode fibers is generallygreater than 10⁻⁴. A highly elliptical core and stress-inducing membersin the cladding region are two main techniques to induce largebirefringence. In non-birefringent fibers, the effective indexdifference between polarization states is generally smaller than 10⁻⁵.

An acoustic wave propagation member 18 has a distal end 20 that iscoupled to optical fiber 12. Acoustic wave propagation member 18propagates an acoustic wave from a proximal end 22 to distal end 20 andlaunches a flexural wave in optical fiber 12. The flexural wave createsa periodic microbend structure in optical fiber 12. The periodicmicrobend induces an antisymmetric refractive index change in the fiberand, thereby, couples light in the fiber from different modes travelingwithin optical fiber 12 such as a core mode to cladding modes. Forefficient mode coupling, the period of the microbending, or the acousticwavelength, should match the beatlength between the coupled modes. Thebeatlength is defined by the optical wavelength divided by the effectiverefractive index difference between the two modes.

Acoustic wave propagation member 18 can be mechanically coupled to theoptical fiber and minimizes acoustic coupling losses in between theoptical fiber and the acoustic wave propagation member. In oneembodiment, acoustic wave propagation member 18 is coupled to opticalfiber 12 in a manner to create a lower order mode flexure wave inoptical fiber 12. In another embodiment, acoustic wave propagationmember 18 is coupled to the optical fiber to match a generation of modescarried by optical fiber 12.

Acoustic wave propagation member 18 can have a variety of differentgeometric configurations but is preferably elongated. In variousembodiments, acoustic wave propagation member 18 is tapered proximal end22 to distal end 20 and can be conical. Generally, acoustic wavepropagation member 18 has a longitudinal axis that is parallel to alongitudinal axis of optical fiber 12.

At least one acoustic wave generator 24 is coupled to proximal end 22 ofacoustic wave propagation member. Acoustic wave generator 24 can be ashear transducer.

Acoustic wave generator 24 produces multiple acoustic signals withindividual controllable strengths and frequencies. Each of the acousticsignals can provide a coupling between different modes traveling withinthe fiber. Acoustic wave generator 24 can produce multiple acousticsignals with individual controllable strengths and frequencies. Each ofthe acoustic signals provides a coupling between different modestraveling within optical fiber 12. A wavelength of an optical signalcoupled to cladding 16 from core 14 is changed by varying the frequencyof a signal applied the acoustic wave generator 24.

Acoustic wave generator 24 can be made at least partially of apiezoelectric material whose physical size is changed in response to anapplied electric voltage. Suitable piezoelectric materials include butare not limited to quartz, lithium niobate and PZT, a composite of lead,zinconate and titanate. Other suitable materials include but are notlimited to zinc monoxide. Acoustic wave generator 24 can have amechanical resonance at a frequency in the range of 1-20 MHz and becoupled to an RF signal generator.

Referring now to FIG. 2, one embodiment of acoustic wave propagationmember 18 has an interior with an optical fiber receiving channel 26.Channel 26 can be a capillary channel with an outer diameter slightlygreater than the outer diameter of the fiber used and typically in therange of 80˜150 microns. The length of the capillary channel ispreferably in the range of 5˜15 mm. The interior of acoustic wavepropagation member 18 can be solid. Additionally, acoustic wavepropagation member 18 can be a unitary structure.

Optical fiber 12 is coupled to acoustic wave propagation member 18. Asillustrated in FIG. 3(a), the dimensions of channel 26 and an outerdiameter of optical fiber 12 are sufficiently matched to place the twoin a contacting relationship at their interface. In this embodiment, therelative sizes of optical fiber 12 and channel 26 need only besubstantially the same at the interface. Further, in this embodiment,the difference in the diameter of optical fiber 12 and channel 26 are inthe range of 1˜10 microns.

In another embodiment, illustrated in FIG. 3(b), a coupling member 28 ispositioned between optical fiber 12 and channel 26 at the interface.Suitable coupling members 28 including but are not limited to bondingmaterials, epoxy, glass solder, metal solder and the like.

The interface between channel 26 and optical fiber 12 is mechanicallyrigid for efficient transduction of the acoustic wave from the acousticwave propagation member 18 to the optical fiber 12.

Preferably, the interface between optical fiber 12 and channel 26 issufficiently rigid to minimize back reflections of acoustic waves fromoptical fiber 12 to acoustic wave propagation member 18.

In the embodiments of FIGS. 3(a) and 3(b), acoustic wave propagationmember 18 is a horn that delivers the vibration motion of acoustic wavegenerator 24 to optical fiber 12. The conical shape of acoustic wavepropagation member 18, as well as its focusing effect, providesmagnification of the acoustic amplitude at distal end 20, which is asharp tip. Acoustic wave propagation member 18 can be made from a glasscapillary, such as fused silica, a cylindrical rod with a central hole,and the like.

In one embodiment, a glass capillary is machined to form a cone and aflat bottom of the cone was bonded to a PZT acoustic wave generator 24.Optical fiber 12 was bonded to channel 26. Preferably, distal end 20 ofacoustic wave generator 18 is as sharp as possible to minimizereflection of acoustic waves and to maximize acoustic transmission.Additionally, the exterior surface of acoustic wave generator 18 issmooth. In another embodiment, acoustic wave generator 18 is a horn witha diameter that decreases exponentially from proximal end 22 to distalend 20.

As illustrated in FIG. 4, filter 10 can also include an acoustic damper30 that is coupled to optical fiber 12. Acoustic damper 30 includes ajacket 32 that is positioned in a surrounding relationship to opticalfiber 12. Acoustic damper 30 absorbs incoming acoustic waves andminimizes reflections of the acoustic wave. The reflected acoustic wavecauses an intensity modulation of the optical signal passing through thefilter by generating frequency sidebands in the optical signal. Theintensity modulation is a problem in most applications. A proximal end34 of the acoustic damper 30 can be tapered. Acoustic damper 30 can bemade of a variety of materials. In one embodiment, acoustic damper 30 ismade of a soft material that has a low acoustic impedance so thatminimizes the reflection of the acoustic wave. Jacket 32 itself is asatisfactory damper and in another embodiment jacket 32 takes the placeof acoustic damper 30. Optionally, jacket 32 is removed from thatportion of optical fiber 12 in a first region 36 and that portion ofoptical fiber 12 that is bonded to acoustic wave generator 24.

First region 36 is where there is a coupling between any two or moremodes within the fiber by the acoustic wave. Examples of couplingbetween two fiber modes includes, core to core, core to cladding andpolarization to polarization. This coupling is changed by varying thefrequency of a signal applied to acoustic wave generator 24. In oneembodiment, first region 36 extends from distal end 20 to at least aproximal portion within acoustic damper 30. In another embodiment, firstregion 36 extends from distal end 20 and terminates at a proximal end ofacoustic damper 30. In one embodiment, the length of optical fiber 12 infirst region 36 is less than 1 meter, and preferably less than 20 cm.The nonuniformity of the fiber reduces the coupling efficiency and alsocauses large spectral sidebands in the transmission spectrum of thefilter. Another problem of the long length is due to the modeinstability. Both the polarization states of the core and cladding modesand the orientation of the symmetry axis of an antisymmetric claddingmode are not preserved as the light propagates over a long lengthgreater than 1 m. This modal instability also reduces the couplingefficiency and causes large spectral sidebands. Preferably, the outerdiameter of optical fiber 12, with jacket 32, is in the range of 60-150microns.

The profile of the refractive index of the cross section of opticalfiber 12 influences its filtering characteristics. One embodiment ofoptical fiber 12, illustrated in FIG. 5, has a first and second cladding16′ and 16″ with core 14 that has the highest refractive index at thecenter. First cladding 16′ has an intermediate index and second cladding16″ has the lowest index. Most of the optical energy of severallowest-order cladding modes is confined both only in core 14 and firstcladding 16′. The optical energy falls exponentially from the boundarybetween first and second claddings 16′ and 16″, respectively.

Optical fields are negligible at the interface between second cladding16″ and the surrounding air, the birefringence in the cladding modes,due to polarization-induced charges, is much smaller than inconventional step-index fibers where second cladding 16″ does not exist.The outer diameter of first cladding 16′ is preferably smaller than thatof second cladding 16″, and can be smaller by at least 5 microns. In onespecific embodiment, core 14 is 8.5 microns, first cladding 16′ has anouter diameter of 100 microns and second cladding 16″ has an outerdiameter of 125 microns. Preferably, the index difference between core14 and first cladding 16′ is about 0.45%, and the index differencebetween first and second claddings 16′ and 16″ is about 0.45%.

In another embodiment, the outer diameter of first cladding 16′ issufficiently small so that only one or a few cladding modes can beconfined in first cladding 16′. One specific example of such an opticalfiber 12 has a core 14 diameter of 4.5 microns, first cladding 16′ of 10microns and second cladding 16″ of 80 microns, with the index differencebetween steps of about 0.45% each.

The optical and acoustic properties of optical fiber 12 can be changedby a variety of different methods including but not limited to, (i)fiber tapering, (ii) ultraviolet light exposure, (iii) thermal stressannealing and (iv) fiber etching.

One method of tapering optical fiber 12 is achieved by heating andpulling it. A illustration of tapered optical fiber 12 is illustrated inFIG. 6. As shown, a uniform section 38 of narrower diameter is createdand can be prepared by a variety of methods including but not limited touse of a traveling torch. Propagation constants of optical modes can begreatly changed by the diameter change of optical fiber 12. The pullingprocess changes the diameter of core 14 and cladding 16 and also changesthe relative core 14 size due to dopant diffusion. Additionally, theinternal stress distribution is modified by stress annealing. Taperingoptical fiber 12 also changes the acoustic velocity.

When certain doping materials of optical fiber 12 are exposed toultraviolet light their refractive indices are changed. In oneembodiment, Ge is used as a doping material in core 14 to increase theindex higher than a pure SiO₂ cladding 16. When a Ge-doped optical fiber12 is exposed to ultraviolet light the index of core 14 can be changedas much as 0.1%. This process also modifies the internal stress fieldand in turn modifies the refractive index profile depending on theoptical polarization state. As a result, the birefringence is changedand the amount of changes depends on optical modes. This results inchanges of not only the filtered wavelength at a given acousticfrequency or vice versa but also the polarization dependence of thefilter. Therefore, the UV exposure can be an effective way of trimmingthe operating acoustic frequency for a given filtering wavelength aswell as the polarization dependence that should preferably be as smallas possible in most applications.

Optical fiber 12 can be heated to a temperature of 800 to 1,300° C. orhigher to change the internal stresses inside optical fiber 12. Thisresults in modification of the refractive index profile. The heattreatment is another way of controlling the operating acoustic frequencyfor a given filtering wavelength as well as the polarization dependence.

The propagation velocity of the acoustic wave can be changed bychemically etching cladding 16 of optical fiber 12. In this case, thesize of core 14 remains constant unless cladding is completely etched.Therefore, the optical property of core mode largely remains the same,however, that of a cladding mode is altered by a different claddingdiameter. Appropriate etchants include but are not limited to hydrofluoride (HF) acid and BOE.

The phase matching of optical fiber 12 can be chirped. As illustrated inFIG. 6, a section 40 of optical fiber can have an outer diameter thatchanges along its longitudinal length. With section 40, both the phasematching condition and the coupling strength are varied along its z-axis42 and the phase matching conditions for different wavelengths satisfiedat different positions along the axis. The coupling then can take placeover a wide wavelength range. By controlling the outer diameter as afunction of its longitudinal axis 42, one can design varioustransmission spectrum of the filter. For example, uniform attenuationover a broad wavelength range is possible by an appropriate diametercontrol.

Chirping can also be achieved when the refractive index of core 14 isgradually changed along z-axis 42. In one embodiment, the refractiveindex of core 14 is changed by exposing core 14 to ultraviolet lightwith an exposure time or intensity as a function of position along thelongitudinal axis. As a result, the phase matching condition is variedalong z-axis 42. Therefore, various shapes of transmission spectrum ofthe filter can be obtained by controlling the variation of therefractive index as a function of the longitudinal axis 42.

As illustrated in FIG. 8 a heatsink 44 can be included to cool acousticwave generator. In one embodiment, heatsink 44 has a proximal face 46and a distal face 48 that is coupled to the acoustic wave generator 24.Preferably, acoustic wave generator 24 is bonded to distal face 48 byusing a low-temperature-melting metal-alloy solder including but notlimited to a combination of 95% zinc and 5% tin and indium-based soldermaterials. Other bonding material includes heat curable silver epoxy.The bonding material should preferably provide good heat and electricalconduction. Heatsink 44 provides a mount for the acoustic wave generator24. Heatsink can be made of a variety of materials including but notlimited to aluminum, but preferably is made of a material with a highheat conductivity and a low acoustic impedance.

Acoustic reflections at proximal face can be advantageous if controlled.By introducing some amount of reflection, and choosing a right thicknessof heatsink 44, the RF response spectrum of acoustic wave generator 24can be modified so the overall launching efficiency of the acoustic wavein optical fiber can be less dependent on the RF frequency.

In this case, the reflectivity and size of heatsink 44 is selected toprovide a launching efficiency of the flexural wave into optical fiber12 almost independent of an RF frequency applied to acoustic wavegenerator 24. The thickness of heatsink 44 is selected to provide atravel time of an acoustic wave from distal face 48 to proximal face 46,and from proximal face 46 to distal face 48 that substantially matches atravel time of the acoustic wave traveling through acoustic wavepropagation member 24 from its proximal end to its distal end, and fromits distal end to its proximal end. The heat sink material or thematerial for the attachment to the proximal face 46 is selected toprovide the amount of back reflection from the heat sink thatsubstantially matches the amount of back reflection from the acousticwave propagation member. In various embodiments, the proximal and distalfaces, 46, 48 of heatsink 44 have either rectangular or circular shapeswith the following dimensions: 10×10 mm² for the rectangular shape anddiameter of 10 mm for the cylindrical shaped heat sink.

However, acoustic back reflections due to proximal face 46 arepreferably avoided. Acoustic reflections from the heat sink back to theacoustic wave generator are reduced by angling proximal face 46 at anangle greater than 45 degree or by roughing the face. The acoustic wavecoming from the acoustic generator toward the angled proximal face 46 isreflected away from the acoustic generator, reducing the acoustic backreflection to the acoustic wave generator. The roughed face also reducesthe acoustic reflection by scattering the acoustic wave to randomdirections. Preferably, the side faces of the heat sink are alsoroughened or grooved to scatter the acoustic wave and thereby to avoidthe acoustic back reflection. Another method to reduce the backreflection is to attach an acoustic damping material at the proximalface 46. Suitable materials that reduce back reflections include softpolymers, silicone, and the like that can be applied to proximal face46.

Referring again to FIG. 8, an acoustic damper mount 50 supports acousticdamper 30. Acoustic damper mount 50 can be made of a variety ofmaterials including but not limited to silica, invar, and the like. Afilter mount 52 supports heatsink 44 and acoustic damper mount 50. Inone embodiment, filter mount is a plate-like structure. Preferably,filter mount 52 and optical fiber 12 have substantially the same thermalexpansion coefficients. Filter mount 52 and fiber 12 can be made of thesame materials.

Filter mount 52 and optical fiber 12 can have different thermalexpansion coefficients and be made of different materials. In oneembodiment, filter mount 52 has a lower thermal expansion coefficientthan optical fiber 12. Optical fiber 12 is tensioned when mounted andbonded to the filter mount 52. The initial strain on optical fiber 12 isreleased when the temperate increases because the length of filter mount52 is increased less than optical fiber 12. On the other hand, when thetemperature decreases optical fiber 12 is stretched further. When theamount of strain change according to temperature change is appropriatelychosen by selecting proper material for mount the 52, the filteringwavelength of filter 10 can be made almost independent of temperature.Without such mounting arrangement, the center wavelength of the filterincreases with temperature. Additionally, first region 36 of issufficiently tensioned to compensate for changes in temperature of firstregion 36 and filter mount 52.

In another embodiment, illustrated in FIG. 9, a filter housing 54encloses first region 36. Filter housing 54 can be made of a variety ofmaterials, including but not limited to silica, invar and the like.Filter housing 54 eliminates the need for a separate filter mount 52.Filter housing 54 extends from distal face 48 of heatsink 44 to acousticdamper 30 or to a jacketed portion 32 of optical fiber 12. Acoustic wavepropagation member 18, acoustic wave generator 24 and the acousticdamper 30 can be totally or at least partially positioned in an interiorof filter housing 54.

In one embodiment, filter housing 54 and optical fiber 12 are made ofmaterials with substantially similar thermal expansion coefficients. Asuitable material is silica. Other materials are also suitable andinclude invar. Filter housing 54 and optical fiber 12 can have differentthermal expansion coefficients and be made of different materials. Inone embodiment, filter housing 54 has a lower thermal expansioncoefficient than optical fiber 12.

In one embodiment, first region 36 is sufficiently tensionedsufficiently to compensate for changes in temperature of first region 36and filter housing 54.

As illustrated in FIG. 10, filter 10 can be a component or subassemblyof an optical communication system 56 that includes a transmitter 58 anda receiver 60. Transmission 58 can include a power amplifier with filter10 and receiver 60 can also include a pre amplifier that includes filter10. Additionally, optical communication system 56 may also have one ormore line amplifiers that include filters 10.

Referring now to FIG. 11, if an electric signal 57 with constantfrequency “f” is applied to acoustic wave generator 24, a flexuralacoustic wave having the same frequency “f” is generated. The flexuralacoustic wave is transferred to optical fiber 12 and propagates alongoptical fiber 12, finally absorbed in acoustic damper 30. The flexuralacoustic wave propagating along optical fiber 12 produces periodicmicrobending along the fiber, resulting in the periodic change ofeffective refractive index which the optical wave propagating alongoptical fiber 12 experiences. The signal light propagating along opticalfiber 12 in a core mode can be converted to a cladding mode by thechange of effective refractive index in optical fiber 12.

When signal light is introduced into filter 10 part of the signal lightis converted to a cladding mode due to the effect of the acoustic waveand the remainder of the signal light propagates as a core mode whilethe signal light propagates along first region 36. The signal lightconverted to a cladding mode cannot propagate any longer in opticalfiber 12 with jacket 32 because the light is partly absorbed in opticalfiber 12 or partly leaks from optical fiber 12. A variety of modeselecting means, including a mode conversion means between core modesand cladding modes, can be incorporated in filter 10. For example, thelong-period grating described in the article “Long-period fiber-gratingbased gain equalizers” by A. M. Vengsarkar et al. in Optics Letters,Vol. 21, No. 5, p. 336, 1996 can be used as the mode selecting means. Asanother example, a mode coupler, which converts one or more claddingmodes of one fiber to core modes of the same fiber or another fiber, canalso be used.

A flexural acoustic wave generated by acoustic wave propagation member18 propagates along first region 36. The acoustic wave createsantisymmetric microbends that travel along first region 36, introducinga periodic refractive-index perturbation along optical fiber 12. Theperturbation produces coupling of an input symmetric fundamental mode toan antisymmetric cladding mode when the phase-matching condition issatisfied in that the acoustic wavelength is the same as the beat lengthbetween the two modes. The coupled light in the cladding mode isattenuated in jacket 32. For a given acoustic frequency, the couplingbetween the fundamental mode and one of the cladding modes takes placefor a particular optical wavelength, because the beat length hasconsiderable wavelength dispersion. Therefore, filter 10 can be operatedas an optical notch filter. A center wavelength and the rejectionefficiency are tunable by adjustment of the frequency and the voltage ofRF signal applied to acoustic wave propagation member 18, respectively.

The coupling amount converted to a different fiber mode is dependent onthe wavelength of the input signal light. FIG. 12(a) shows the couplingamounts as functions of wavelength when flexural acoustic waves at thesame frequency but with different amplitudes are induced in opticalfiber 12. As shown in FIG. 12(a), the coupling amounts are symmetricalwith same specific wavelength line (λ_(c)), i.e.,. center wavelengthline, however they show different results 62 and 64 due to the amplitudedifference of the flexural acoustic waves. Therefore, the transmittanceof the output light which has passed through filter 12 is differentdepending on the wavelength of the input light. Filter 12 can act as anotch filter which filters out input light with specific wavelength asshown in FIG. 12(b).

FIG. 12(b) is a graph showing the transmittances as a function ofwavelength when flexural acoustic waves with different amplitudes areinduced in optical fiber 12. The respective transmittances have samecenter wavelength as does the coupling amount, but differenttransmittance characteristic 64 and 66 depending on the amplitudedifference of the flexural acoustic waves can be shown.

The center wavelength λ_(c), of filter 10 satisfies the followingequation.

β_(co)(λ)−β_(cl)(λ)=2π/λ_(a)

In the above equation, β_(co)(λ) and β_(cl)(λ) are propagation constantsof core mode and cladding mode in optical fiber 12 which arerespectively dependent on the wavelength, and λ_(a) represents thewavelength of the flexural acoustic waves.

Accordingly, if the frequency of the electric signal applied to acousticwave generator 24 varies, the wavelength of the acoustic wave generatedin optical fiber 12 also varies, which results in the center wavelengthchange of filter 10. In addition, since the transmission is dependent onthe amplitude of the flexural acoustic wave, the transmission of signallight can be adjusted by varying the amplitude of the electric signalwhich is applied to acoustic wave generator 24.

FIG. 13 is a graph showing the transmittance of filter 10 in oneembodiment when different electric signal frequencies are applied. Asshown in FIG. 13, each center wavelength (i.e., wavelength showingmaximum attenuation) of filter 10 for different electric signals was1530 nm, 1550 nm and 1570 nm. Therefore the center wavelength of filter10, according to the embodiment, is changed by varying the frequency ofthe electric signal which is applied to acoustic wave generator 24.

As described above, since there are a plurality of cladding modes infirst region 36 the core mode can be coupled to several cladding modes.FIG. 14 is a graph showing the center wavelength of filter 10 accordingto the embodiment of the invention as a function of the frequencyapplied to the flexural acoustic wave generator. In FIG. 14, straightlines 71, 72 and 73 represent the center wavelength of filter 10resulting from the coupling of a core mode with three different claddingmodes.

Referring to FIG. 14, there are three applied frequencies for any oneoptical wavelength in this case. Therefore the input signal light isconverted to a plurality of cladding modes by applying multi-frequencyelectric signal to acoustic wave generator 24. Moreover, it meanstransmission characteristics of filter 10 can be electrically controlledby adjusting the amplitude and each frequency component of the electricsignal.

As shown in FIG. 15(a), the respective transmission features 74, 75 and76 of filter 10 can be provided by applied electric signals withdifferent frequencies f1, f2 and f3. In this example, assuming that f1couples the core mode of input signal light to a cladding mode (claddingmode A), f2 couples the core mode to other cladding mode (cladding modeB) and f3 couples the core mode to another cladding mode different fromA or B (cladding mode C, the transmission feature is shown in FIG. 15(b)as a curve 77 when electric signal with three frequency components f1,f2 and S is applied to acoustic wave generator 24.

As shown in FIG. 5(c), if filter 10 has transmission feature curves 78,79 and 80 corresponding to respective frequencies f1′, f2′ and f3′ andelectric signal having three frequency components f1′, f2′ and f3′ isapplied to the flexural acoustic wave generator, the transmissionfeature of filter 10 is shown as a curve 81 of FIG. 5(d).

FIGS. 16(a) and 16(b) are graphs showing the transmittance of filter 10according to an embodiment of the present invention, when varyingelectric signal having three frequency components is applied to filter10. When varying electric signal having a plurality of frequencycomponents is applied to acoustic wave generator 24 various shapes oftransmittance curves 82, 83 and 84 can be obtained.

Since conventional tunable wavelength filters utilize the coupling ofonly two modes, the difference between a plurality of appliedfrequencies naturally becomes small to obtain wide wavelength bandfiltering feature by applying a plurality of frequencies. In this case,as described under the article “Interchannel Interference inmultiwavelength operation of integrated acousto-optical filters andswitches” by F. Tian and H. Herman in Journal of Light wave technology1995, Vol. 13, n 6, pp. 1146-1154, when signal light input to a filteris simultaneously converted into same (polarization) mode by variousapplied frequency components, the output signal light may undesirably bemodulated with frequency corresponding to the difference between theapplied frequency components. However, with filter 10 the above problemcan be circumvented, because the respective frequency components convertthe mode of input light into different cladding modes in filter 10.

In one embodiment, the filtering feature shown in FIG. 17(a) wasobtained by applying adjacent frequencies 2.239 MHz and 2.220 MHz toreproduce the result of a conventional method. The applied twofrequencies were such that convert the mode of input light into the samecladding mode. Under the condition, narrow wavelength-band signal lightwith a center wavelength of 1547 nm was input to filter 10 to measureoutput light. Referring to the measurement result shown in FIG. 17(b),there is an undesirable modulated signal with frequency corresponding tothe difference of the two applied frequencies.

In another embodiment, when adjacent frequencies 2.239 MHz and 2.220 MHzwere applied to acoustic wave generator 24, according to the embodimentof the invention, the two frequency components convert the mode of inputlight into mutually different cladding modes. FIG. 17(c) shows themeasurement result when the same signal light as the above experimentwas input to filter 10 and output light was measured.

However, an undesirable modulated signal, which appeared in aconventional filter, practically disappeared as shown in FIG. 17(d).

In optical communications or optical fiber sensor systems, wavelengthfilters are required that has a wide tuning range and are capable ofelectrically controlling its filtering feature.

FIG. 18(a) illustrates one embodiment of a transmission spectrum offilter 10 with a 15.5-cm-long interaction length for a broadbandunpolarized input light from a LED. A conventional communication fiberwas used with a nominal core diameter of 8.5 μm, a cladding outerdiameter of 125 μm and a normalized index difference of 0.37%. Thefrequency of the applied RF signal was 2.33 MHz, and the correspondingacoustic wavelength was estimated to be ˜650 μm. The three notches shownin FIG. 8(a) are from the coupling to three different cladding modeswith the same beat length at the corresponding wavelengths. The coupledcladding modes were the LP₁₁ ^((cl)), the LP₁₂ ^((cl)), and the LP₁₂^((cl)) modes, which was confirmed from far-field radiation patterns.The center of each coupling wavelength was tunable over>100 nm by tuningthe acoustic frequency.

FIG. 18(b) shows the measured and the calculated center wavelengths ofthe notches as a function of acoustic frequency. The fiber parametersused in the calculation for best fit with the experimental results are acore diameter of 8.82 μm, a cladding outer diameter of 125 μm, and anormalized index difference of 0.324%, in reasonable agreement with theexperimental fiber parameters.

Referring again to FIG. 8(a), coupling light of a given wavelength fromthe fundamental mode to different cladding modes requires acousticfrequencies that are separated from each other by a few hundredkilohertz. This separation is large enough to provide a widewavelength-tuning range of almost 50 nm for each coupling mode pairwithout significant overlap with each other, thereby practicallyeliminating the coherent cross talk that is present in conventionalcounterparts. The tuning range is sufficient to cover the bandwidth oftypical EDFA's. In one embodiment, filter 10 provides for a combinationof independent tunable notch filters built into one device, and thenumber of involved cladding modes corresponds to the number of filters.The multifrequency acoustic signals can be generated by a singletransducer, and the spectral profile of filter 10 is determined by thefrequencies and amplitudes of the multiple acoustic signals.

FIG. 19 shows two examples of the configurable spectral profiles withspectral tilt, which can be used to recover the gain flatness in an EDFAwith a gain tilt caused by signal saturation. In one embodiment, threecladding modes [LP₁₁ ^((cl)), the LP₁₂ ^((cl)), and the LP₁₃ ^((cl))]were used and three RF signals were simultaneously applied withdifferent voltages and frequencies adjusted for the particular profile.The 3-dB bandwidth of the individual notch was ˜6 nm with a 10-cm-longinteraction length.

A complex filter profile is required to flatten an uneven EDFA gain,which exhibits large peaks with different widths around 1530 and 1560nm. The combination of three Gaussian shaped passive filters can producea flat gain over a 30-nm wavelength range.

As illustrated in FIG. 20(a), a filter assembly of the present inventioncan include first and second filters 10′ and 10″ in series. Each filter10′ and 10″ is driven by three radio frequency (RF) signals at differentfrequencies and amplitudes that produce acousto-optic mode conversionfrom the fundamental mode to different cladding modes. This approacheliminates the detrimental coherent crosstalk present in LiNbO₃-basedAOTF's. The 3-dB bandwidths of the first filter 10′ were 3.3, 4.1, and4.9 nm for the couplings to the cladding modes LP₁₂ ^((cl)), the LP₁₃^((cl)), and LP₁₄ ^((cl)), respectively. For second filter 10″, theywere 8, 8.6, and 14.5 nm for the couplings to the cladding modes, LP₁₁^((cl)), the LP₁₂ ^((cl)), and the LP₁₃ ^((cl)) respectively.

The minimum separations of notches produced by single RF drivingfrequency were ˜50 nm for first filter 10′ and ˜150 nm for second filter10″, respectively, so that only one notch for each driving frequencyfalls into the gain-flattening range (35 nm). The large differencebetween filters 10′ and 10″ was due to the difference in optical fiber12 outer diameters. The polarization splitting in the center wavelengthof the notches as ˜0.2 nm for first filter 10′ and ˜1.5 nm for secondfilter 10″. The relatively large polarization dependence in secondfilter 10″ is due mainly due to the unwanted core elliptically andresidual thermal stress in optical fiber 12, that can be reduced to anegligible level by using a proper optical fiber. First and secondfilters 10′ and 10″ were used for the control of the EDFA gain shapearound the wavelengths of 1530 and 1555 nm, respectively. The backgroundloss of the gain flattening AOTF was less than 0.5 dB, which was mainlydue to splicing of different single-mode fibers used in the two AOTF's1010. Adjusting the frequencies and voltages of the applied RF signalsprovided control of the positions and depths of the notches with greatflexibility. The RF's were in the range between 1 and 3 MHz.

FIG. 20(b) shows a schematic of a dual-stage EDFA employing gainflattening filter 10 along with a test setup. A 10-m-long EDF pumped bya 980-nm laser diode and a 24-m-long EDF pumped by a 1480-nm laser diodewere used as the first and the second stage amplifiers, respectively.The peak absorption coefficients of both EDF's were ˜2.5 dB/m at 1530nm. Filter 10 was inserted between the two stages along with anisolator. Total insertion loss of filter 10 and the isolator was lessthan 0.9 dB. Six synthesizers and two RF power amplifiers were used todrive filter 10.

Gain profiles of the EDFA were measured using a saturating signal at thewavelength of 1547.4 nm and a broad-band light-emitting diode (LED)probing signal. The saturating signal from a distributed feedback (DFB)laser diode was launched into the EDFA after passing through aFabry-Perot filter (optical bandwidth: 3 GHz, extinction ratio: 27 dB)to suppress the sidelobes of the laser diode. The total power of theprobe signal in 1520-1570-nm range was 27 dBm, which is much smallerthan that of the input saturating signal ranging from 13 to 7 dBm.

FIG. 21(a) shows gain profiles before and after the gain flattening fortwo different saturating signal powers of 13 and 7 dBm when thesecond-stage pump power was 42 mW. The gain excursions before flatteningwere larger than 5 dB. By adjusting the filter profile, flat gainprofiles within 0.7 dB were obtained over 35 nm for both cases. The flatgain region is shifted slightly toward the shorter wavelength for highergain level, which is due to the intrinsic gain characteristics of theEDF. FIG. 21(b) shows filter profiles that produced the flat gainprofiles shown in FIG. 21(a), where Profile 1 and Profile 2 are for thecases of saturating tones of 13 and 7 dBm, respectively. For themeasurements, EDFI was used as an ASE source, while the second pumpdiode (1480 nm) was turned off. The ASE signal leaked out of the secondWDM coupler was monitored and the signals obtained when the filter wason and off were compared to yield the filter response. The attenuationcoefficients for Profile 1 and Profile 2 at the saturating signalwavelength were 5.0 and 4.9 dB, respectively, and the averageattenuation over the 35-nm range (1528-1563 nm) was 5 dB in both cases.The total RF electrical power consumption of the filter was less than500 mW. Profile 2 could be obtained from Profile 1 by adjusting mainlythe depths of notches, although fine adjustments of center wavelengthsof notches within 0.5-nm range slightly improved the gain flatness. FIG.21(c) shows the filter profiles of first filter 10′ and second filter10″ used to form Profile 1, and also the locations of center wavelengthsof six notches. By adjusting first and second filters 10′ and 10″spectral profiles electronically gain flatness of <0.7 dB over 35-nmwavelength range were obtained at various levels of gain as well asinput signal and pump power.

One important characteristic of filter 10 is polarization dependence.The shape of filter 10 can be dependent on the polarization state ofinput light. The polarization dependence originates from fiberbirefringence. Fiber birefringence causes the effective propagationconstant of a mode to be different between two eigen polarizationstates. Since the magnitude of birefringence is different from mode tomode, the fiber birefringence causes the beat length between two coupledmodes to be different between the eigen polarization states, and,therefore, results in splitting of center wavelength of filter 10 for agiven acoustic frequency.

FIG. 22(a) illustrates the polarization dependence. Curve 90 representsthe filter profile for light in one eigen polarization state, and filterprofile 92 is when the input is in the other eigen state. The centerwavelengths are split because of the birefringence. Moreover, since thefield overlap between two coupled modes is also polarization dependentdue to the birefringence, the attenuation depth can be different betweenfilter profiles 90 and 92.

A critical feature due to the polarization dependence is thepolarization dependent loss (PDL) which is defined as the difference ofthe magnitude of attenuation between two eigen polarization states.Since polarization dependent loss is an absolute value, it increaseswith the attenuation depth. FIG. 22(b) shows polarization dependent lossprofile 93 associated with filter profiles 90 and 92. In WDMcommunication system applications, the polarization dependent lossshould be minimized. Most applications require the polarizationdependent loss to be less than 0.1 dB. However, acousto-optic tunablefilter 10 has exhibited a typical polarization dependent loss as largeas 2 dB at 10-dB attenuation level. This is due to the largebirefringence of the antisymmetric cladding modes.

FIG. 23 shows one possible configuration that can reduce the inherentpolarization dependent loss of filter 10. In FIG. 23, double-pass filter100 consists of a 3-port circulator with input-, middle-, andoutput-port fibers, 12′, 12″ and 12′″, respectively, and Faradayrotating mirror (FRM) 104. The middle-port fiber 12″ is connected toacousto-optic tunable filter 10 and Faraday rotating mirror 104. Whenlight comes in through input-port fiber 12′, it is directed to filter10, through circulator 102, and then refracted by Faraday rotatingmirror 104. Faraday rotating mirror 104 acts as a conjugate mirror withrespect to optical polarization states. So, if the light pass throughfilter 10 in a specific polarization state, then on the way back afterreflection it pass through filter 10 in its orthogonal polarizationstate. Since the light pass through filter 10 twice but in mutuallyorthogonal states, the total attenuation after the double pass becomespolarization-insensitive. Another benefit of the double passconfiguration is that, since the filtering takes place twice in filter10, the drive RF power applied to filter 10 to obtain a certainattenuation depth is reduced half compared to single-pass configurationas in FIG. 4. For instance, when filter 10 is operated at an attenuationdepth of 5 dB, the overall attenuation depth of double-pass filter 100becomes 10 dB.

Another embodiment of a device configuration for low polarizationdependence is shown in FIG. 24. In this embodiment, dual filter 110consists of filters 10′ and 10″ in tandem and connected through midfiber section 112. Filters 10′ and 10″ are preferably operated at thesame RF frequency. The birefringence of mid fiber section 112 isadjusted such that it acts as a half-wave plate aligned with 45-degreeangle with respect to the eigen polarization axes of filters 10′ and10″. In other words, the light passing through filter 10′ in one eigenpolarization state enters filter 10″ in the other eigen polarizationstate. If the polarization dependent loss is the same loss for filters10′ and 10″, the overall attenuation after passing through filters 10′and 10″ becomes polarization-insensitive. If filters 10′ and 10″ are notidentical in terms of polarization dependent loss, the double filter 110would exhibit residual polarization dependent loss that should be,however, smaller than the polarization dependent loss of individualfilters, 10′ or 10″. Therefore, it is desirable that filters 10′ and 10″are identical devices. Since the filtering takes place by two filters,the drive powers to individual filters are reduced, compared to using asingle filter alone, to achieve the same attenuation depth.

In one embodiment, illustrated in FIG. 23, circulator 102 based onmagneto-optic crystal has overall insertion loss and polarizationdependent loss of 1.5 dB and 0.5 dB, respectively. Faraday rotatingmirror 104 has insertion loss and polarization dependent loss of 0.5 dBand 0.5 dB, respectively. Curve 120 in FIG. 24(a) shows the polarizationdependent loss profile in one embodiment when filter 10 was operated toproduce 10 dB attenuation at 1550 nm. The filter profile in thisinstance is shown by curve 124 in FIG. 24(b). Optical fiber 12 used inthe filter was a conventional communication-grade single mode fiber.When the filter was used in the double-pass configuration, the overallpolarization dependent loss was reduced greatly as shown by curve 121 inFIG. 24(a).

The polarization dependent loss was reduced down to less than 0.2 dB.The total insertion loss of double-pass filter was 3 dB, mainly due tothe circulator and splices. In this embodiment, the drive power tofilter 10 required to produce total 10-dB attenuation at 1550 nm, asshown by filter profile 125 in FIG. 24(b), was decreased compared to thesingle-pass filter experiment.

In another embodiment, illustrated in FIG. 25, two filters werefabricated with a conventional circular-core single mode fiber. Eachfilter was operated with 5-dB attenuation at the same center wavelength,1550 nm. The overall dual filter profile is shown by curve 126 in FIG.24(b). In these filters, the eigen polarization states are linear andtheir axes are parallel and orthogonal to the direction of the flexuralacoustic wave vibration or the acoustic polarization axis. This isgenerally true with filters made of a circular-core fiber where thedominant birefringence axes are determined by the lobe orientation ofthe cladding mode, which is the same as the acoustic polarization axis.Linear axis orthogonal to acoustic polarization is the slow axis, andits orthogonal axis is the fast axis. In this embodiment, a polarizationcontroller was used in mid fiber section 112 and controlled to minimizethe overall polarization dependent loss of dual filter 110.

The loss profile is shown by curve 122 in FIG. 24(a). The total filterprofile is shown by curve 126 in FIG. 24(b). The residual polarizationdependent loss as large as 0.6 dB is primarily due to differentpolarization dependent loss of filters 10′ and 10″, and could be reducedgreatly if identical two filters were used.

Another important characteristic of filter 10 is the intensitymodulation of an optical signal passing through the filter. One reasonwhich gives rise to the intensity modulation of the output signal isstatic coupling between different modes traveling within the fibereither by microbending of fiber 12 or imperfect splices, if present.Another reason is an acoustic wave propagating backward in first region36 by an acoustic reflection at imperfect acoustic damper 30 and fiberjacket 32. FIG. 26 shows an example of output signal 139 suffering fromthe intensity modulation by backward acoustic reflection at acousticdamper 30. In this case, the major modulation frequency is equal totwice the acoustic frequency. The modulation depth is defined by theratio of peak-to-peak AC voltage amplitude, V_(AC) to DC voltage,V_(DC). By static mode coupling, the major modulation frequency is equalto the acoustic frequency. When both static mode coupling and backwardacoustic wave are present, the intensity of the output is modulated atfrequencies of both first- and second-harmonics of the acousticfrequency. The modulation depth, when smaller than 20%, is,approximately, linearly proportional to the amount of attenuation in dBscale. In most WDM communication system applications, the modulationdepth is generally required to be less than 3% at 10-dB attenuationlevel.

In one embodiment, illustrated in FIG. 4, filter 10 was fabricated byusing a conventional single-mode fiber. The modulation depth at 10-dBattenuation level was about 10% at both first- and second-harmonics ofthe acoustic frequency, as shown by curves 140 and 141 in FIG. 27(a),respectively. The same filter was used as filter 10 in anotherembodiment, illustrated in FIG. 23. The RF drive power to the filter wascontrolled to produce 10-dB attenuation depth. In the first embodimentof double-pass filter 100, the length of fiber section 106 was selectedsuch that the round-trip travel time of fiber section 106 is equal to aquarter of the period of the acoustic wave. In this case, thesecond-harmonics component of the intensity modulation can becompensated out. Curves 142 and 143 in FIG. 27(a) show the modulationdepth of first- and second-harmonics components, respectively. Thesecond-harmonics was eliminated almost completely. The first-harmonicswas also reduced a little, which may be attributed to imperfect lengthmatching of fiber section 106. In the second embodiment of double-passfilter 100, the length of fiber section 106 was such that the opticalround-trip travel time of fiber section 106 is equal to a half of theperiod of the acoustic wave. In this case, the first-harmonics componentof the intensity modulation can be reduced. Curves 147 and 148 in FIG.27(b) show the modulation depth of first- and second-harmonicscomponents, respectively. The first-harmonics was eliminated almostcompletely.

Reduction of intensity modulation can also be achieved by dual filter110 where the length of mid fiber section 112 is selected properly. Forexample, if the first-harmonic modulation component is to becompensated, the length of mid fiber section 112 is such that theoptical travel time from one end of section 112 to the other end isequal to a half of the period of the acoustic wave.

Referring now to FIG. 28, one embodiment of the present invention is atunable VOA assembly 150 that includes a tunable VOA 152, coupled to atap coupler 154, a detector 156 and an attenuator control circuit 158that creates a local loop. A network loop is created by couplingattenuator control circuit 158 to network components in order to receivenetwork commands. Attenuator 152 can be a liquid crystal attentuator, aMEMS device, an acoustic-optic device, a Fabry Perot device, amechanical sliding attenuator, a magneto-optic device and the like.

Tap coupler 154 can be a fused directional coupler, a bulk optic filter,a grating positioned in a fiber, and the like. Detector 156 can be anyphotodetector well known to those skilled in the art.

VOA 152 couples light from a fundamental core mode of an optical fiberto a higher-order mode such as a higher order core mode or a claddingmode. The configuration of VOA 152 is preferably the same as AOTF 10.The amount of coupling is determined by the amplitude of the acousticwave. Transmission in the fundamental core mode is controlled by thevoltage of an RF signal applied to the transducer.

Optical tap coupler 154, optical power detector 156, and an attenuatorcontrol circuit 158 provide a feedback loop to VOA 152. Additionally,the feedback signal can come from other system elements, not shown, thatare coupled with VOA system 150. Control circuit 158 can include adecision circuit and an RF generator. Control circuit 158 compares theoutput signal power determined from the detector output with a targetvalue required by a system operator. Control circuit 158 controls thevoltage of the RF signal that goes to the transducer of VOA 152 so thatthe optical signal power approaches the target value.

VOA assembly 150 is suitable with single or multiple wavelength channelsand/or bands, depending on the wavelength bandwidth of the AO couplingand the channel and/or band spacing. VOA assembly 150 provides broadbandoperation, spectral attenuation and broadband tilt adjustment. VOAassembly 150 can provide approximate flat spectral attenuation or it canprovide a tilt adjustment by moving the match to one side or the other.This can require network feedback from a spectral monitor. Further, twoVOA assemblies 150 can be in series, with one providing tile and theother overall attenuation.

Referring now to FIG. 29, a channel equalizer 159 is illustrated. Whenused for a single wavelength channel, VOA 152 is likely to be positionedin an optical node incorporating a demultiplexer 160, such as an arrayedwave-guide grating (AWG) router. For example, VOA 152 can be used toequalize the powers of multiple channels and/or bands including, inparticular, added channels and bands. In this case, the RF frequency foreach VOA 152 is set differently according to the attenuation desired foreach channel and/or band VOA 152 is to deal with. Polarizationdependence of each VOA 152 is largely tolerated due to the feedbackoperation as long as the feedback speed is faster than the polarizationfluctuations. For example, the characteristic time of SOP fluctuation ina real communication system can be on the order of a millisecond.

Demultiplexer 160 is configured to receive a plurality of different WDMchannels and separate the different signals (channels) into differentfibers, one fiber for each wavelength channel and/or bands. Eachseparate wavelength is individually attenuated with a VOA 152. Thisprovides individual control for each wavelength. Some of the wavelengthscan be dropped and not passed to VOA's 152. New wavelengths can be addedafter demultiplexer 160. VOA's 152 provide gain flattening and alsopermit adding and dropping of channels and/or bands. VOA's 152 providespectral flattening and adjust the powers so the recombined signals allhave a predetermined power.

In the FIG. 29 embodiment, a first series of VOA's 152 is positionedbetween the drop and add and a second series of VOA's 152 positionedafter the add. A multiplexer 162 is positioned downstream from thesecond series of VOA's 152. A monitor 164 is coupled to the output.Monitor 164 can provide a feedback signal that is used to adjust VOA's152. The embodiment of FIG. 29 provides broadband operation, spectralattenuation, channel by channel spectral attenuation and broadband tiltadjustment.

Referring now to FIG. 30, one embodiment of an optical cross-connectapparatus 166 is provided. Optical cross-connect apparatus 166 provideschannel routing, switching and leveling between two inputs with multiplewavelengths and two outputs of multiple wavelengths. Opticalcross-connect apparatus 166 includes demultiplexer 160, multiplexer 162,a demultiplexer 168 and a multiplexer 170 and coupled to optical crossconnect 172 which includes any number of devices to redirectwavelengths, including but not limited to mirrors and the like. Opticalcross connect 172 can be made using MEMS mirrors, bubble switchtechnology, with liquid crystals and the like. A plurality of VOA's 152are each coupled to an optical fiber and positioned between opticalcross connect 172 and multiplexers 162 and 170. VOA's 152 are includedto individually adjust the power of individual wavelengths and achieveleveling and/or spectral grooming.

Optionally included is a monitor 174 which can be a spectral monitor andthe like that monitors the spectral output. A system command device 176is coupled to monitor 174 to receive network commands and create afeedback loop. These network commands can, (i) come from the end of thelink after electrical detection and bit error rate measurements, (ii)come from spectral monitors located throughout the network and (iii) canbe IP signals or proprietary signals to the local controlelectronics/processor. A second monitor 178 is provided at the secondoutput. The embodiment illustrated in FIG. 30 also provides broadbandoperation, spectral attenuation and channel by channel broadbandspectral adjustment.

Referring now to FIG. 31, another embodiment of an optical cross-connectapparatus 180 includes two or more optical cross connects 172 and 182.At least two demultiplexers 160 and 182 are provided at the inputcarrying WDM signals. At least two multiplexers 162 and 184 are at theoutput. The wavelengths are split into two groups. All of the evenwavelengths are in one group and the odd wavelengths in the other group.One group is directed to optical cross connect 172 and the other groupis directed to optical cross connect 182. Thereafter, multiplexers 162and 184 combine the different wavelengths which are directed to thedifferent output fibers. A plurality of VOA's 152 are coupled to opticalcross connects 172 and 182. Spectral monitors 174 couple the outputfibers with VOA's 152. Amplifiers 186 can be coupled to the opticalfibers carrying the WDM signals. It will be appreciated that theembodiment of FIG. 31 can be extended to any desired number of opticalcross connects, demultiplexers and multiplexers.

FIG. 32 illustrates another embodiment of the present invention. In thisembodiment filter 210 includes optical fiber 212 with first region 214and second region 216. There is more coupling of the optical signalbetween modes traveling within the optical fiber in first region 214than in second region 216. First acoustic wave generator 218 is coupledto optical fiber 212. First acoustic wave generator 218 produces a firstacoustic wave that travels in a first direction 220 in first region 214.In response to propagation of the first acoustic wave in first region214, a backward-propagating wave is created and travels in an oppositedirection 222 to the first acoustic wave. A first acoustic wavepropagation member 226 is coupled to optical fiber 212. A secondacoustic wave generator 228 is coupled to optical fiber 212 at secondregion 216. Second acoustic wave generator 228 produces a secondacoustic wave that reduces a magnitude of the backward propagatingacoustic wave.

Optical fiber 212 can include a cladding and a core. An optical signalis coupled to the cladding from the core in first region 214. Afrequency of the first acoustic wave is preferably the same as thefrequency of the second acoustic wave. The second acoustic wave is outof phase with the backward propagating acoustic wave. The secondacoustic wave is preferably 90 to 270 degrees out of phase with thebackward propagating acoustic wave, more preferably about 180 degreesout of phase.

The second acoustic wave reduces a power of the back reflection in arange of 20 to 30 db or less, more preferably 30 to 40 db or less, morepreferably 40 to 50 db or less and still more preferably 50 to 60 db orless.

First acoustic wave generator 218 can produce multiple acoustic signalswith individual controllable strengths and frequencies. Each of theseacoustic signals provides a coupling between different modes travelingwithin the fiber. A wavelength of an optical signal coupled to acladding from a core of optical fiber 212 is changed by varying thefrequency of a signal applied to first acoustic wave generator 218. Anamount of an optical signal coupled to a cladding from a core of opticalfiber 212 is changed by varying the amplitude of a signal applied tofirst acoustic wave generator 218.

Referring now to FIG. 33, a feedback loop 230 is coupled to a feedbackand processing unit 231 which is coupled to one or more second acousticgenerators 228. By looking at the various second harmonic signals,feedback signals can be calculated at feedback and processing unit 231and sent to each second acoustic generator 228 to remove substantiallyall of the second harmonic intensity modulation.

As illustrated in FIG. 34, filter 210 can also include an acousticdamper 232 coupled to non-interaction region 216. In this embodiment,acoustic damper 232 has a proximal end 233 with a sufficient taperconfiguration that reduces a power of the backward-propagating wave in arange of 20 to 30 dB or less, preferably 30 to 40 dB or less, morepreferably 40 to 50 dB or less and still more preferably 50 to 60 dB orless.

Referring now to FIG. 35, second acoustic wave generator 228 is coupledto acoustic damper 232. In this embodiment, acoustic damper 232 need nothave the selected tapered configuration. Second acoustic wave generator228 produces the second acoustic wave that reduces the magnitude of thebackward propagating acoustic wave. Additionally, acoustic damper 232can have the tapered configuration in order to help reduce the magnitudeof the backward propagating acoustic wave.

The foregoing description of a preferred embodiment of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formsdisclosed. Obviously, many modifications and variations will be apparentto practitioners skilled in this art. It is intended that the scope ofthe invention be defined by the following claims and their equivalents.

What is claimed is:
 1. An acousto-optic filter, comprising: an opticalfiber with a first region and a second region; a first acoustic wavegenerator coupled to the optical fiber, the first acoustic wavegenerator producing a first acoustic wave that travels in a firstdirection in the first region, wherein a backward-propagating acousticwave is created in response to propagation of the first acoustic wavealong the optical fiber with the backward-propagating acoustic wavetraveling in an opposite direction to the first acoustic wave; a firstacoustic wave propagation member coupled to the optical fiber; and asecond acoustic wave generator coupled to the optical fiber at thesecond region, the second acoustic wave generator producing a secondacoustic wave that is combined with the backward-propagating acousticwave.
 2. The filter of claim 1, wherein the backward propagatingacoustic wave combined with the second acoustic wave reduces a magnitudeof the backward-propagating acoustic wave.
 3. The filter of claim 1,wherein the optical fiber includes a cladding and a core.
 4. The filterof claim 3, wherein an optical signal is coupled to the cladding fromthe core in the first region.
 5. The filter of claim 1, the first regionprovides more coupling of an optical signal between modes travelingwithin the optical fiber than in the second region.
 6. The filter ofclaim 1, wherein a frequency of the first acoustic wave is the same as afrequency of the second acoustic wave.
 7. The filter of claim 1, whereinthe second acoustic wave is out of phase with the backward-propagatingacoustic wave.
 8. The filter of claim 1, wherein the second acousticwave is 90 to 270 degrees out of phase with the backward-propagatingacoustic wave.
 9. The filter of claim 1, wherein the second acousticwave is about 180 degrees out of phase with the backward-propagatingacoustic wave.
 10. The filter of claim 1, wherein the second acousticwave reduces a power of the backward-propagating acoustic wave in anamount of 20 db or less.
 11. The filter of claim 1, wherein the secondacoustic wave reduces a power of the backward-propagating acoustic wavein an amount of 30 db or less.
 12. The filter of claim 1, wherein thesecond acoustic wave reduces a power of the backward-propagatingacoustic wave in an amount of 40 db or less.
 13. The filter of claim 1,wherein the second acoustic wave reduces a power of thebackward-propagating acoustic wave in an amount of 50 db or less. 14.The filter of claim 1, wherein the second acoustic wave reduces a powerof the backward-propagating acoustic wave in an amount of 60 db or less.15. The filter of claim 1, wherein the first acoustic wave generatorproduces multiple acoustic signals with individual controllablestrengths and frequencies and each of the acoustic signals provides acoupling between different modes traveling within the fiber.
 16. Thefilter of claim 1, wherein the first acoustic wave is a longitudinalwave.
 17. The filter of claim 1, wherein the first acoustic wave is atorsional wave.
 18. The filter of claim 1, wherein the first acousticwave is a shear wave.
 19. The filter of claim 1, wherein a wavelength ofan optical signal coupled between two different modes traveling withinthe optical fiber is changed by varying a frequency of a signal appliedto the first acoustic wave generator.
 20. The filter of claim 1, whereinan amount of an optical signal coupled between two different modestraveling within the optical fiber is changed by varying the amplitudeof a signal applied to the first acoustic wave generator.
 21. The filterof claim 1, wherein the first acoustic wave generator produces multipleacoustic signals with individual controllable strengths and frequenciesand each of the acoustic signals provides a coupling between differentmodes traveling within the optical fiber.
 22. The filter of claim 1,further comprising: a feedback loop coupled to the second acousticgenerator to reduce second harmonic intensity modulation.
 23. Anacousto-optic filter, comprising: an optical fiber with a first regionand a second region; a first acoustic wave generator coupled to theoptical fiber, the first acoustic wave generator producing a firstacoustic wave that travels in a first direction in the first region,wherein a backward-propagating wave is created in response topropagation of the first acoustic wave along the optical fiber with thebackward-propagating wave traveling in an opposite direction to thefirst acoustic wave; and an acoustic damper coupled to the optical fiberat a non-interaction region, the acoustic damper including a proximalend with a taper configuration that reduces a power of thebackward-propagating wave in an amount of 10 dB or less.
 24. The filterof claim 23, wherein the taper configuration reduces the power of thebackward-propagating wave in an amount of 20 dB or less.
 25. The filterof claim 23, wherein the taper configuration reduces the power of thebackward-propagating wave in an amount of 30 dB or less.
 26. The filterof claim 23, wherein the taper configuration reduces the power of thebackward-propagating wave in an amount of 40 dB or less.
 27. The filterof claim 23, wherein the taper configuration reduces the power of thebackward-propagating wave in an amount of 50 dB or less.
 28. The filterof claim 23, wherein the taper configuration reduces the power of thebackward-propagating wave in an amount of 60 dB or less.
 29. The filterof claim 23, the first region provides more coupling of the opticalsignal between modes traveling within the optical fiber than in thesecond region.
 30. The filter of claim 23, further comprising: afeedback loop coupled to the second acoustic generator to reduce secondharmonic intensity modulation.
 31. An acousto-optic filter, comprising:an optical fiber with a first region and a second region; a firstacoustic wave generator coupled to the optical fiber, the first acousticwave generator producing a first acoustic wave that travels in a firstdirection in the first region, wherein a backward-propagating acousticwave is created in response to propagation of the first acoustic wavealong the optical fiber with the backward-propagating acoustic wavetraveling in an opposite direction to the first acoustic wave; a firstacoustic wave propagation member coupled to the optical fiber; anacoustic damper coupled to a non-interaction region of the opticalfiber; and a second acoustic wave generator coupled to the acousticdamper, the second acoustic wave generator producing a second acousticwave that reduces a magnitude of the backward-propagating acoustic wave.32. The filter of claim 31, the first region provides more coupling ofthe optical signal between modes traveling within the optical fiber thanin the second region.
 33. The filter of claim 31, wherein a frequency ofthe first acoustic wave is the same as a frequency of the secondacoustic wave.
 34. The filter of claim 31, wherein the second acousticwave is out of phase with the first acoustic wave.
 35. The filter ofclaim 31, wherein the second acoustic wave is 90 to 270 degrees out ofphase with the backward-propagating acoustic wave.
 36. The filter ofclaim 31, wherein the second acoustic wave reduces a magnitude of thebackward-propagating acoustic wave to 20 db or less.
 37. The filter ofclaim 31, wherein the second acoustic wave reduces a magnitude of thebackward-propagating acoustic wave to 30 db or less.
 38. The filter ofclaim 31, wherein the second acoustic wave reduces a magnitude of thebackward-propagating acoustic wave to 40 db or less.
 39. The filter ofclaim 31, wherein the second acoustic wave reduces a magnitude of thebackward-propagating acoustic wave to 50 db or less.
 40. The filter ofclaim 31, wherein the second acoustic wave reduces a magnitude of thebackward-propagating acoustic wave to 60 db or less.
 41. The filter ofclaim 31, wherein the first acoustic wave generator produces multipleacoustic signals with individual controllable strengths and frequenciesand each of the acoustic signals provides a coupling between differentmodes traveling within the fiber.
 42. The filter of claim 31, whereinthe first acoustic wave is a longitudinal wave.
 43. The filter of claim31, wherein the first acoustic wave is a torsional wave.
 44. The filterof claim 31, wherein the first acoustic wave is a shear wave.
 45. Thefilter of claim 31, wherein a wavelength of an optical signal coupledbetween two modes traveling within the optical fiber is changed byvarying a frequency of a signal applied to the first acoustic wavegenerator.
 46. The filter of claim 31, wherein an amount of an opticalsignal coupled between two modes traveling within the optical fiber ischanged by varying the amplitude of a signal applied to the firstacoustic wave generator.
 47. The filter of claim 31, wherein the firstacoustic wave generator produces multiple acoustic signals withindividual controllable strengths and frequencies and each of theacoustic signals provides a coupling between different modes travelingwithin the optical fiber.
 48. The filter of claim 31, furthercomprising: a feedback loop coupled to the second acoustic generator.49. A method, comprising: producing a first acoustic wave to propagatein a first direction in a first region of an optical fiber; creating abackward-propagating acoustic wave in response to propagation of thefirst acoustic wave along the optical fiber, the backward-propagatingacoustic wave propagating in an opposite direction to the first acousticwave; and producing a second acoustic wave in the optical fiber in asecond region of the optical fiber.
 50. The method of claim 49, furthercomprising reducing a magnitude of the backward-propagating acousticwave by combining the second acoustic wave with the backward-propagatingacoustic wave.
 51. The method of claim 49, further comprising providingmore coupling of an optical signal between modes traveling within theoptical fiber in the first region than in the second region.
 52. Themethod of claim 49, wherein producing the second acoustic wave comprisesproducing the second acoustic wave out of phase with thebackward-propagating acoustic wave.
 53. The method of claim 49, whereinproducing the second acoustic wave reduces a power of thebackward-propagating acoustic wave in an amount no more than 20 db. 54.The method of claim 49, further comprising reducing reflections of thebackward-propagating acoustic wave.
 55. The method of claim 49, furthercomprising reducing second harmonic intensity modulation in the opticalfiber.
 56. An acoustic-optic filter, comprising: means for producing afirst acoustic wave to propagate in a first direction in a first regionof an optical fiber; means for creating a backward-propagating acousticwave in response to propagation of the first acoustic wave along theoptical fiber, the backward-propagating acoustic wave propagating in anopposite direction to the first acoustic wave; and means for producing asecond acoustic wave in the optical fiber in a second region of theoptical fiber.
 57. The acoustic-optic filter of claim 56, furthercomprising means for reducing a magnitude of the backward-propagatingacoustic wave by combining the second acoustic wave with thebackward-propagating acoustic wave.
 58. The acoustic-optic filter ofclaim 56, further comprising means for providing more coupling of anoptical signal between modes traveling within the optical fiber in thefirst region than in the second region.
 59. The acoustic-optic filter ofclaim 56, wherein the means for producing the second acoustic wavecomprises means for producing the second acoustic wave out of phase withthe backward-propagating acoustic wave.
 60. The acoustic-optic filter ofclaim 56, wherein the means for producing the second acoustic wavereduces a power of the backward-propagating acoustic wave in an amountno more than 20 db.
 61. The acoustic-optic filter of claim 56, furthercomprising means for reducing reflections of the backward-propagatingacoustic wave.
 62. The acoustic-optic filter of claim 56, furthercomprising means for reducing second harmonic intensity modulation inthe optical fiber.